Imaging using penetrating radiation has been applied for the generation of images using a variety of techniques. The oldest technique produces an image (typically on X-ray film, although not necessarily) which is referred to as a shadowgraph. Because of the disadvantages of shadowgraphs, i.e. it gives a line of sight projection of the object whose image has been produced, improvements have long been sought. Tomography, i.e. the production of an image representing a slice or plane through an object being examined, has been a long-felt need. Techniques are available for the production of tomographic images or tomograms, although the presently known techniques all have various drawbacks.
One of those techniques is illustrated by Olendorf in U.S. Pat. No. 3,106,640. He mentions complicated mechanical techniques such as planigraphy, tomography, laminography and the like, which typically involve the synchronous motion, at identical angular rates, of a radiation source and a recording plane which is sensitive to the radiation. In principle, motion during the exposure period theoretically blurs everything not on a plane parallel to the plate and on the axis of rotation. This provides a sectional radiograph (or tomograph) of a layer of the object under consideration, which layer has a vague and indefinite thickness. Thus, the mechanical isolation of the layer or section from other detail is quite poor. Other techniques similar to Olendorf are shown by Ander U.S. Pat. No. 3,979,594; Brunnett U.S. Pat. No. 3,976,885; Matsuda U.S. Pat. No. 3,973,127 and deLuca U.S. Pat. No. 3,935,462.
Since the 1970's, a far more popular tomographic technique and one which has achieved widespread success is referred to as computed tomography, relying in the main on the work of Hounsfield and Cormack, for which they jointly received a Nobel prize. While there are a variety of specific techniques comprehended under the term "computed tomography", for our purposes they can all be described together. In computed tomography, an object is illuminated by a beam of penetrating radiation (in some cases this is a fan beam illuminating the object from edge to edge, in other cases it might be one or a plurality of pencil beams). A detector or detectors records the X-ray energy emitted by the object as a result of the illumination. In some fashion or another, the angular relation between the object and the source/detector arrangement is altered and the process is repeated. This is repeated a relatively large number of times, so that the X-ray energy transmitted through the plane of interest is recorded through a plurality of paths and at a plurality of angles. This data is then used as the input to a computer which generates a cross-sectional image of the section of the object that had been illuminated.
Notwithstanding the popularity of computed tomography, there are a number of major disadvantages with this technique. Firstly, in order to obtain the desired image, the object must be illuminated from edge to edge and the X-ray response of the object throughout this region must be recorded. In other words, even if we are aware that our interest is centered on a particularly known region of a known slice of the object, the entire slice must be illuminated and the data taken from that entire slice must be used before we can produce the image that we desire. Secondly, the image is not produced in real time and in fact there is a delay while the computer portion of the equipment operates on the data that has been recorded before the image is available. The necessity for the rotation poses a number of disadvantages. In a very practical sense it limits the slice orientations that are available. Consider for example the human body. Obtaining a slice which is essentially horizontal with respect to a vertically standing individual requires that relative rotation be effected about an axis which is vertical; that is the typical configuration of most popular machines. However, if one happens to desire a vertical slice relative to a vertical individual, then the human body must be rotated about a horizontal axis, i.e. the human body must be rotated head to foot. We do not know of any practical machines which are capable of this type of rotation. A significant limitation of computed tomography is the fact that the contrast at any region of the image is ultimately limited by the statistical fluctuation or noise in that region. It is a characteristic of CT that if there is a large attenuation of an X-ray beam traversing any part of the image, the entire image becomes more noisy.
A device for producing tomographic images which does not exhibit the prior art disadvantages is described in copending application Ser. No. 888,019, filed July 22, 1986 and assigned to the assignee of this application. The device described in application Ser. No. 888,019 uses a flying spot beam illuminating radiation and a line collimator; the subject matter of the above-referenced application is incorporated herein by this reference. The image produced by the device described in the copending application comprising the image of a slice is made up of a sequence of line images, each representing the radiant energy response of that portion of the object being imaged lying along a line within the slice. By providing relative motion between the object being imaged and the source/detector assembly, an image of the entire slice is produced. It is a distinct advantage of the apparatus described in the copending application that the only motion required for the production of the tomographic image is that motion to bring different "lines" of the object into the field of view of the collimator. On the other hand, the apparatus described in the copending application has a disadvantage in that images produced are subject to degradation caused by multiple scattering events in the object being imaged as a consequence of its illumination by the penetrating radiant energy.
Accordingly, it is an object of the invention to provide a device capable of producing tomographic images which does not require the computing power required by computed tomography and which has the signal to noise ratio advantages over the computed tomography similar to those exhibited by the apparatus of the copending application but which is more resistant to image degradation caused by multiple scattering.
It is another object of the present invention to provide for tomographic imaging using penetrating radiant energy which does not require the complex motion typical of computed tomography and which does not require the object being imaged to be illuminated edge to edge as is required in computed tomography.
These and other objects of the invention will become more apparent in the course of the following description.